1. Field of the Invention
The invention relates to a method for fabricating a semiconductor device, and more particularly to a method for fabricating an oxide-semiconductor (OS) transistor.
2. Description of the Prior Art
Attention has been focused on a technique for formation of a transistor using a semiconductor thin film formed over a substrate having an insulating surface. The transistor is used in a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor, and within which, oxide semiconductor containing indium (In), gallium (Ga), and zinc (Zn) has been attracting attention.
A transistor including an oxide semiconductor film is known to have an extremely low leakage current in an off state. Nevertheless, current architecture of a so-called oxide-semiconductor transistor is still insufficient in providing multiple Vt options and allowing flexible device Ion/Ioff. Hence, how to improve the fabrication as well as structure of current oxide-semiconductor device has become an important task in this field.